Determining the Value of the Ideal Gas Constant

Teacher Notes

This procedure is written up supposing you have a balance that will read to at least mg. If you have only a cg balance, then find the mass of a longer piece of Mg (approximately 1 m) and calculate the mass of the shorter pieces by ratio from this value. This gives acceptable results, provided the Mg ribbon is reasonably consistent.
Calculate the approximate mass required by measuring the barometric pressure before the experiment. Each piece must be selected so that it will not produce more than 50 mL of gas. You can calculate the maximum length using this formula: Maximum mass = 4.4 x 10^-4 P where P is in kPa (this formula calculates the mass required to produce 45 mL of gas, allowing a slight safety margin). Multiply by 3 to get the length required to do three trials.
Make the plastic scoop by cutting a 5 mL Berol plastic pipet lengthwise, leaving a small bowl at the end. Then fold over the stem and insert it into the glass tubing connected to the clamp. The pipet will stay in place from friction. The bowl needs to be deep enough that the Mg won’t fall out too easily, but not so deep that you can’t shake it out into the acid when you want to.